One hundred and eighty-one cases of primary small intestinal lymphoma were diagnosed in residents of Israel--145 Jews and 36 Arabs--during a 16-year period, 1960--75. The male:female ratio was 1.8:1. Incidence rates were higher in children, the middle-aged and the elderly than in teenagers or young adults. Crude and age-adjusted incidence rates were higher in Arabs than in Jews. The pattern among the subgroups of the Jewish population varied with age. In young adults of both sexes the rates were much higher among Jews born in North Africa or Asia than among those born in Europe or Israel. In elderly males, however, the rates were almost twice as high in European-born Jews than among those born in North Africa or Asia. The mean annual incidence of primary small intestinal lymphoma fell from 4.8 per million during 1960--67 to 3.6 per million during 1968--75. This was due to a marked fall in the rates in children and young adults, whereas the rates in those aged 40 years or more rose with time. The pattern of the change with time suggests that environmental conditions are an important factor in the causation of primary intestinal lymphoma in young adults.